1. Field of the Invention
The present invention generally relates to a digital line circuit device provided in a subscriber system of a communications system, and more particularly to a signaling-system switching device for selecting one of signaling systems of the digital line circuit device so that a signaling process can be selected which is suitable for the signaling system of a subscriber line circuit extending between a subscriber terminal and an exchange via the digital line circuit device.
2. Description of the Prior Art
FIG. 1A shows a first conventional digital line circuit device DLC1 provided in a subscriber system between a subscriber terminal and an exchange that are connected to each other via a subscriber line circuit. The digital line circuit device DLC1 includes a signal processing block BLK and a multiplexer/demultiplexer MUX/DMUX. The signal processing block BLK includes a first signaling system circuit 1A, and a second signaling system circuit 1B. The first signaling system circuit 1A executes a first signaling process suitable for a first signaling system (an A signaling system) of the subscriber line circuit. The second signaling system circuit 1B executes a second signaling process suitable for a second signaling system (a B signaling system) of the subscriber line circuit. In accordance with the actual signaling system of the subscriber line circuit, either the first signaling system circuit 1A or the second signaling system circuit 1B is selected and connected to the subscriber line circuit by means of a pair of manual switches SW1 and SW2. PCM (Pulse Code Modulated) data is transferred via the subscriber line circuit.
FIG. 1B shows a second digital line circuit device DLC2, which comprises a remote terminal RT and a central office terminal COT. The remote terminal RT is provided at the subscriber side, and is connected to a subscriber terminal, such as a telephone set TEL or a private branch exchange PBX. The central office terminal COT is provided at the side of an exchange 4, and is connected thereto.. The digital line circuit device DLC2 also includes the first and second signaling system circuits (not shown) as shown in FIG. 1A.
Switching between the first and second signaling system circuits is automatically carried out under control of a system controller 5, such as a personal computer. The system controller 5 sends the central office terminal COT switching information indicating the signaling system circuit to be selected from among the first and second signaling system circuits. The switching information is transferred to the remote terminal RT via the subscriber line circuit.
The first digital line circuit device DLC1 shown in FIG. 1A has a disadvantage in that either the first signaling system circuit or the second signaling system circuit must be manually selected by a service provider before services for the subscriber line circuit can be started.
The second digital line circuit device DLC2 shown in FIG. 1B has a disadvantage in that use of the system controller 5 increases the cost for building the communications system and requires additional space in the central office.